Phase-locked far infrared laser

ABSTRACT

A technique for stabilizing a gas laser to a frequency standard, and more particularly to the use of a phase lock loop in which the laser and its power supply are disposed, with the output of the laser being arranged to be compared with a multiplied frequency standard. In this invention the laser output is mixed with the multiplied frequency standard in a mixer, with the beat frequency from the mixer being compared to a radio frequency reference in a phase detector. The output of the phase detector is sent to a current controller which varies the laser power supply current, thus selectively changing the frequency of the laser. If the laser fluctuates in phase, a control signal brings the beat frequency signal back in phase with the reference input to the phase detector. As a result, a hydrogen cyanide laser phase locked in accordance with this invention has produced an output signal whose beat is less than 50 Hz wide, with a long term stability of less than 10 Hz.

455-609 AU 233 EX United Stat 1 X {00 5 y [151 3,689,851

Corcoran et al. Sept. 5, 1972 [54] PHASE-LOCKED FAR INFRARED PrimaryExaminer-Ronald l... Wibert LASER Assistant Examiner-Edward S. Bauer[72] Inventors: Vincent J. Corcoran, Orlando, Fla; Attorney-JulianRenfro and Gay Chin Richard E. Cupp, Boulder, Colo.;

James J. Gallagher, Orlando, Fla. [57] ABSTRACT I [73] Assignee: MartinMarietta Corporation, A technique for stabilizing a gas laser to afrequency New York standard, and more particularly to the use of a phaselock loop in which the laser and its power supply are [22] filed: 1970disposed, with the output of the laser being arranged [21] A l, N 82,256to be compared with a multiplied frequency standard. in this inventionthe laser output is mixed with the multiplied frequency standard in amixer, with the g 5 331/945 iig g beat frequency from the mixer beingcompared to a d 3 6 5 radio frequency reference in a phase detector. Theie 0 Sean: output of the phase detector is Sent to a current com trollerwhich varies the laser power supply current,

[56] References Cited I thus selectively changing the frequency of thelaser. if UNITED STATES PATENTS the laser fluctuates in phase, a controlsignal brings the beat frequency signal back in phase with the 354833812/1970 2 Kmoshlta at 331 94 5 reference input to the phase detector. Asa result, a hydrogen cyanide laser phase locked in accordance QTHERPUBLICATlONS with this invention has produced an output signal whosebeat is less than 50 Hz wide, with a long term Corcoran et al.: IEEEJournal of Quantum Elecstability ofless than 10 HZ tronics, vol. QE- 5,pp. 424- 426, August, 1969 6 Claims, 4 Drawing Figures NP IIDOASVNTMESIIER 10.990251 29 as): um 10 m1 "3 "P nmnenc: nrrtxr m 14.221614m. l m vnm'on 12mm I "I5! HUL'IPLIER CRYSTAL LOCK 0mm. in 40:21 I as soPM evnrqzssoz n5 LOCK In: I I 10 IN: '90 nuns-r an;

wane: mass m e n llULTIPLlEI-HIXEI E W Li (m5!) uzmus um i 1435mm umFOINY-CONYICY IUL'II'USR llm HIXER T use: I nus: vzs career! unrcroacummn uvunrn courmuu FEL 1} SYIICHRONIIEI l I20 El P'ATE'NTEDSEP 51912sum 2 Br 4 INVENTORS VINCENTJ.CORCORAN RICHARD E. CUPP JAMES J.GALLAGHER k??? ATTORNEY INVENTORS VINCENTJ.CORCORAN RICHARD E. CUPPJAMES J. GALLAGHER ATTORNEY PATENTEDSEP 5:912

SHEEI3UF4 FIG. 3

FATENTEDSEP 51972 SHEET 8 OF 4 WWW FIG. 4

INVENTORS VINCENT J. CORCORAN ATTORNEY PHASE-LOCKED FAR INFRARED LASERBACKGROUND OF THE INVENTION Whereas other laser stabilization techniqueshave entailed stabilizing a laser by the use of another laser, whichitself is inherently unstable, and lasers have been stabilized bytechniques in which the laser frequency is varied so that only long termstability results, this novel and inventive technique is the first inwhich a laser is stabilized to an inherently stable device so that longand short term stabilization results and the bandwidth of the lasersignal is actually narrowed.

There were three major problem areas we encountered in stabilizing a gaslaser so that it could be used as a frequency standard. First, we had toget a larger signal out of our mixer, that is, it had to have a morefavorable signal-to-noise ratio. Then, we had to passively stabilize theHCN laser we used, which involved eliminating sources of vibration,mechanical and acoustical, and the current fluctuations in the powersupply which caused the frequency of the HCN laser to vary. Thirdly, wehad to find a current controller capable of taking the signal from thephase detector and which was capable of varying the current in the HCNlaser power supply so that by the functioning of the feedback loop, thefrequency could be controlled.

We achieved the goal of obtaining a more favorable signal to noise ratioout of the mixer by increasing the power output of the HCN laser and bycontinuing runins of a point contact multiplier we used as the mixer.The effectiveness of a point contact multiplier requires application ofthe art of pointing and running in until the desired signal is produced.This operation is conducted by the operator turning a micrometer typedevice that moves a silicon post with respect to a whisker. A signal ona scope indicates when the signal is of sufficient amplitude. The signalto noise ratiofor the 27.36695 MHz IF signal we used was typicallygreater than to l, which is the desirable minimum signal to noise rationeeded for phase locking.

We then determined the sources of instability of the HCN laser bybeating its output with the output of another HCN laser. In this way wecould determine whether mechanical or acoustical vibrations, orfluctuations in the power supply were the sources of the instability. Wefound that mechanical vibrations were an important source of instabilityand we reduced these by mounting the HCN laser tube on a prestressedconcrete block and isolating the block frdm the floor of the buildingwith inflated tubes. Acoustical vibrations were not considered to be amajor source of instability.

Current fluctuations in the power supply were reduced primarily by usinga large LCR filter at the output of the supply. Thus, by mechanicalisolation and electrical filtering, we were able to reduce thefluctuations in the output of the HCN laser to a point where the signalcould be used in an active control system to phase lock the laser. Thatis, the signal remained within the range of operation of the FEL 13Asynchronizer we used.

The third step that was necessary in order that the phase locking of theHCN laser could be accomplished was the constructing of a CurrentController that could be used to change the current through the gasdischarge when the laser changed frequency or phase. This wasaccomplished by operating a control device in series with the laser andpower supply. The device was basically a common emitter transistorcircuit with the collector load in series with the power supply andlaser tube. The input control signal was fed to the base of the circuittransistor so that small changes in control signal could influence thecurrent in the transistor collector circuit and hence through the gasdischarge.

SUMMARY OF THE INVENTION A phase lock loop in accordance with thisinvention involves a gas laser having a power supply and a dischargetube in which a gas discharge is produced, and through which a currentflows, with the power supply being capable of supplying variable currentso that the laser output can be varied in frequency. Current controllermeans are provided for the power supply, for controlling the currentflow through the laser, and phase detector means are utilized forgenerating a control signal substantially proportional to the differencein phase of two input signals to said phase detector. One of these inputsignals is a stable reference, and the other is a beat frequency signalout of a mixer means, with the mixer means being arranged to receive theoutput of the gas laser and also a signal generated from a frequencystandard. The mixer produces therefrom the beat frequency output signal,whose frequency is the difference between the laser signal frequency andthe standard signal frequency, which output signal is delivered to saidphase detector means to be compared with the reference signal. Thecontrol signal from the phase detector means is of course delivered tothe current controller means so as to closely control the current flowof said laser, thus assuring a frequency stable output.

It is a principal object of our invention to provide a phase lockedlaser capable of serving as a frequency standard; and as a stable,narrow linewidth source for transmitting and receiving applications.

It is another object of our invention to provide a phase lock loopusable in conjunction with a gas laser, serving to stabilize the outputof the laser in a highly satisfactory manner.

It is yet another object of our invention to provide a technique formodulating the output of gas lasers which have the characteristic ofproducing output signals whose frequencies change as current through thelasers changes.

The latter technique is capable of producing and a multiplied frequencystandard to a stable reference in a phase detector;

FIG. 3 is a cross sectional view of the point contact multiplieremployed as the mixer in our apparatus; and FIG. 4 is a perspective viewof the point contact multiplier.

DETAILED DESCRIPTION The basic technique for the phase lock of a gaslaser in accordance with our invention is illustrated in FIG. 1, whereinthe significant components of the control loop are shown by means of ablock diagram. The gas laser is generally indicated at 12, with itsoutput being directed into mixer 14. The beat generated in the mixer 14between the laser and the output 16 of a multiplied frequency standardis amplified by IF amplifier 18 if necessary, and compared in a phasedetector 20 to a stable reference 22. It should be noted that if thesignal level out of the mixer 14 is large enough for the phase detectorto operate in its linear region, then the amplifier 18 is not needed.

The output of the phase detector 20 is amplified and filtered by DCamplifier 24, and then provided to a current controller 26. However, theDC amplifier 24 is not needed if the phase detector output is largeenough to drive the current controller. When the beat frequency is notin phase with the reference signal 22, then a correction voltage is fedfrom the phase detector 20 to current controller 26, which is connectedto the laser power supply 28, so that any change in the phase detectoroutput changes in the proper direction, the current flowing through thegas discharge. The current change in the laser discharge changes theoutput frequency of the laser 12. and thus, by the functioning of ourapparatus, the beat frequency output is caused to go back in phase withthe stable reference 22. In this preferred embodiment the laserfrequency was 890,758.735 MHz.

It will be noted in FIG. 2 that we have there shown an exemplary blockdiagram representative of the operational components associated with thephase locking of a gas laser in accordance with out invention. In thisarrangement, the output signal from the hydrogen cyanide laser 112 ismixed with a standard frequency, which in this instance was generated bymultiplying the signal from a phase locked OKI70V11 klystron 116 in apoint contact multiplier 114. Although this constituted our specificstandard frequency, when we refer to a standard frequency or frequencystandard we mean this term to comprehend and include either a fixedfrequency signal, or a short term stable signal that is tunable orsettable to discrete frequencies.

The point contact multiplier device is discussed in detail inconjunction with FIG. 3, and as will be seen, it also acts as the mixer,thus generating the beat between this multiplied standard frequencyrepresented by the klystron output, and the laser signal. The beatfrequency signal was compared to a reference signal in a synchronizer120, which provides the internal reference signal for the phase detectorportion of this device, and the IF amplification for the beat frequencysignal coming from the mixer 114. Although a specific reference signalwas provided in this particular instance, as shown in FIG. 2. when werefer to a stable reference we mean the term to comprehend and includeeither a fixed frequency radio frequency signal or a radio signal whosefrequency spectrum is reasonably pure, but which is tunable or settableto a discrete frequency. For the exemplary version of our invention, wepreferred to use a synchronizer made by the Frequency EngineeringLaboratories of Farmingdale, N.J., Model FELI 3A.

When the FELI 3A synchronizer is used for phaselocking, the internalamplification of the device is sufficient so that the output of thepoint contact multiplier is fed directly into the phase detector portionof the device 120. The phase detector in the FELIBA synchronizerproduces a signal voltage which is proportional to the phase fluctuationbetween the beat frequency signal generated in the mixer 114, and thereference frequency in the synchronizer 120. The output of the phasedetector portion of the synchronizer is then fed to a DC amplifier 124which amplifies the volt age from the phase detector to a levelsufficient to properly operate the current controller 126.

The amplifier 124 may consist of three cascaded stages of KZXAoperational amplifiers which are manufactured by Philbrick Researchers,Inc., Boston, and the combination has a gain of approximately 25, withthe high frequency 3 db point occurring below 10 KHz.

This amplifier may also filter out high frequency components of thephase detector output.

The current controller 126 is used to change the current through the gasdischarge in response to a signal input to the device. In the embodimentof the device according to FIG. 2, the current controller circuit is a2N5385 transmitter in a common emitter configuration with the collectorload connected to the laser tube. The signal on the base of thetransistor controls the operating point of the transistor which has asmall variation in current for variable loads and fixed base current.The details of the current controller circuit are shown in Volume QE-6,March 1970 of the IEEE Journal of Quantum Electronics on page 160. Othermethods may be used for current control and in fact, another circuit isshown in the Journal of Quantum Electronics on page 241 of Volume QE-6,April 1970.

The current controller 126 is in series ,with. ,the laser power supply128 and hydrogen cyanide laser 112 so that any changes in the phasedetector output change the current through the hydrogen cyanide gasdischarge. The current changes thus' change the frequency of the laserin such a direction that the laser output is caused to be in phase withthe multiplied standard frequency.

In the preferred embodiment shown in FIG. 2, we used an HCN laserutilizing a 4 inch inner diameter, 7 foot long Kimax tempered glass pipeto form the discharge tube 140. Concentric, water cooled. stainlesssteel cathode 142 and anode 144, which are of physically the sameconstruction, are connected to the discharge tube. Mirror mounts 146 and148 that house the resonator reflectors are connected to the cathode andanode. Three or so 1 inch invar rods 150 are used to stabilize thedimension of the resonator by connecting the mirror mounts 146 and 148together. These mirror mounts 146 and 148 support the mirrors 152 and154, respectively, each of which is a spherical brass mirror with a 20foot radius of curvature. The output mirror 154 has a 0.275 inchdiameter hole in the center to couple out the radiation.

The mixture used to generate the HCN was ethyl ether and ammonia, whichare continuously added through the port 158 disposed in the outputmirror mount 148. An electrical current from power supply 128 is causedto pass between cathode and anode, and this creates a gas discharge, oneof whose constituents Hun-mil is HCN. This mixture flows through the gasdischarge tube, and out through an outlet port 156 on the rear mirrormount 146. A Welch 1402 fore pump with a long suction line continuouslypumps the mixture through the tube and out from the port 156. The anodeis grounded and the cathode is at a negative voltage, in the range of800 1200 V, and it is for this reason that the long suction line betweenthe HCN laser and the fore pump is needed, so that the voltage from thecathode does not discharge through the fore pump.

A mechanical coupling involving a gear train 160 was made to the rearmirror 152. This coupling can be used to translate the mirror 152,thereby tuning the length of the resonator to the desired frequency ofthe HCN laser. The tuning can be performed by hand, or a drive motor canbe utilized.

A waveguide coupler 162, which may be a R0135 waveguide, may be used totake the radiation from the output mirror 154, and to connect the l-lCNoutput to the point contact multiplier 114. A quartz window 164 adjacentthe mirror mount transmits the radiation to the waveguide coupler andallows the low pressure of from 50 microns to l millimeter of mercury tobe maintained in the laser tube.

Other gas laser tubes can be used, and more specifically, a number ofHCN laser configurations would be satisfactory to produce the neededoutput frequency. Obviously, the diameter and length of the dischargetube can be changed. Various metals other than stainless steel have beenused as cathode and anode, not only in concentric configurations butwith a hollow tube design in a side arm as well. Mirrors may be mountedexternal to the gas discharge as well as internal. When mirrors aremounted externally, Brewster windows made of polypropylene or othermaterials having low absorption and high transmission in the wave lengthrange of interest can be used on the ends of the discharge tube. Otherstable materials such as Cer- Vit can be used in place of lnvar for thestabilizing rods of the laser.

The entire laser is preferably mounted on a prestressed concrete blockor the like, and the block isolated from the floor with air filled innertubes, thus to reduce mechanical vibrations. Other techniques such asmechanically isolated tables can be used to reduce the mechanicalvibration.

1n the preferred embodiment illustrated in FIG. 2, the signal from theklystron 116 was stabilized to a 120 MHz crystal 170 through amultiplier chain which incorporated two phase lock loops. In the firstphase lock loop, the signal from the crystal 170 was multiplied by 69 ina MA 4052 Varactor multiplier 172. This signal, along with the signalfrom an X -13 klystron l74, is fed to an X band commercially availablecross guide coupler 176 to which a 1N23 mixer 178 is connected, with thesignal being mixed with the signal from the klystron in the mixer. Thebeat frequency from the mixer 178 was then fed to the phase detectorportion 180 of a Dymec 2660A synchronizer 182 by a lead 184. The beatfrequency signal was compared in device 182 to a 29.0335 Ml-lz referencesignalthat was derived from a Hewlett Packard 5100A synthesizer 186. Anoutput voltage is generated in the Dymec 2660A synchronizer which isproportional to the fluctuation in phase of the beat frequency signalcompared to the 29.0335 MHz reference. This control voltage is then fedto the reflector of the X-l 3 klystron 174 to bring the beat frequencysignal in phase with the 29.0335 Ml-lz reference from synthesizer 186.

A portion of the stabilized X-band signal is coupled out of the crossguide coupler 176 to a IN 5 3 multiplier mixer 190. In this latterdevice the X-band signal is multiplied by 9 and mixed with the74,227.614 MHz signal from the klystron 116. The difference frequencythat is generated at 30 MHz is fed to a Schomandl FDS 30 Phase Lock 192,otherwise known as a Syncriminator. The 30 MHz signal is compared with areference in the Phase Lock device that is created in the phase lockdevice, by multiplying a 10 MHz reference by three, and a voltage isgenerated in this device which is proportional to the difference inphase between the reference and the signal from the 1N53 multipliermixer 190. This voltage is then fed to the reflector of the klystron 116to bring the beat frequency signal in phase with the reference in thePhase Lock 192. In this way, the klystron 116, whose function wasdiscussed in conjunction with FIG. 2, has been stabilized to the signalfrom the MHz crystal 170.

Referring now to FIG. 3, it will be seen that we have there shown ingreater detail the point contact multiplier 114 utilized in accordancewith our invention. The essential elements of the device are a crystalor chip 202 and a tungsten whisker 204. Although in the preferredembodiment we utilized for chip 202, a piece cut from the crystal of acommercial 1N23 diode, a wide variety of materials may be used, such asgallium arsenide, indium antimonide, germanium, or other pieces ofsilicon. The tungsten whisker 204, which is preferably electronics gradetungsten wire, can also be selected from a variety of metal wires,including phosphor bronze. As will be obvious to one skilled in the art,tungsten wire is most often used with a silicon chip, and phosphorbronze wire is typically used with a gallium arsenide chip.

1n the point contact multiplier we used, the tungsten whisker 204 isconnected electrically to a commercially available coaxial BNC connector206 through the whisker mount 208 shown in the drawing. Member 208 ispreferably brass and the whisker is soldered to an upraised tip 210 ofthe member 208. The member 208 in turn is supported by a Rexolite sleeve212 that serves to insulate the member 208 from the housing 214. Acylindrical brass sleeve may surround sleeve 212, being used as amounting for the sleeve, and simplifying assembly procedures. It shouldbe noted that a mica washer 216 is used to insulate the upper portion ofmember 208 from the wall of waveguide 218, which is a RG 98 waveguideused to couple the output from the klystron 116 into the multiplier 114.

The connector member 206 may be regarded as extending upwardly intocontact with member 208. More particularly, the center conductor of theBNC is electrically and mechanically in contact with a recess in thelower portion of member 208. The illustrated BNC Connector 206 of courseis a female member designed to receive a male member that in turnreceives the output from the multiplier-mixer 114, and delivers it tothe device 120.

It should be noted that chip 202 can be caused to move with respect towaveguide 162, which is the RG 135 waveguide connected from the laser112, in which waveguide a hole has been provided, into which the chipmay be moved.

Contact between the tungsten whisker 204 and the silicon chip 202 ismade by means of the differential screw 224 to the lower end of whichthe silicon chip is mounted. In other devices that we have used, thetungsten whisker is mounted to the differential screw; eitherconfiguration is usually satisfactory.

The differential screw operates when the knurled outer knob 226 isrotated. The screw 224 has two different threads, a coarse thread 228that is threadedly received in member 230 that is fixed to housing 214,and a fine thread 232 that is threadedly received in post mountingmember 234, that can move vertically as shown in F IG. 3, withoutrotating. Thus, rotation of the knob in one direction moves the assemblytoward the whisker through the action of the coarse thread 228. At

the same time, the fine thread 232 draws the post mounting member 234and the chip 202 away from the whisker. By this conventionaldifferential screw arrangement we obtain very fine mechanical adjustmentof the chip with respect to the whisker.

Energy from the microwave and laser sources is coupled into theinteracting region by means of the crossed waveguide arrangement, withthe hole through the crossed area being sufficient in size to allow thepassage of the point contact junction. As shown in FIGS. 3 and 4,waveguides 162 and 218 cross at a right angle, with the waveguide 162being used to couple the HCN laser signal, and the waveguide 218 beingthat through which energy from the 70Vl l klystron .116 is coupled.

Turning now to FIG. 4, it will be noted that we have illustrated thepoint contact multiplier 114 so as to reveal the manner in which thewaveguides cross, and the use of adjustment knobs 240 and 242 thatenable plungers (not shown) in each of waveguides 218 and 162 to bemoved. This of course makes it possible to tune the signals by settingup standing waves in the waveguides. This is to say, the plungers aremanipulated until the interaction of the signal from the K1 70Vllklystron 116 into the wire or whisker is maximized. This is of courseaccomplished after the point contact multiplier has been run-in.

The device shown in our FIGS. 3 and 4 is essentially a state of the artcomponent, variously known as a harmonic generator, as used in F 1G. 12of Spectroscopy at Radio and Microwave Frequencies by D. J. E. Ingram(Butterworths Scientific Publications, London 1955); as a multiplier, asused on pages 50 and 51 of Microwave Spectroscopy by Gordy, Smith andTrambarulo (John Wiley and Sons 1953 and as a mixer. No claim toinvention by us is made herein.

Although we have shown and described our invention in conjunction withcertain devices, certain frequencies, and the like, we are not to belimited to these, and for example, in place of the klystrons 116 and174, we can use carcinotrons or even certain solid state devices.

Also, in place of the 120 MHz crystal 170, we can use any one of anumber of stable sources including atomic frequency sources, such as forexample rubidium, or masers, such as an ammonium gas maser.

We claim:

l. in a gas laser whose output frequency depends on the amount ofcurrent flow through the gas discharge of said laser, a power supply, acurrent controller for controlling the flow of current out of said powersupply and through the gas discharge of said laser, a phase detector forproducing a signal which is proportional to the phase difference betweena stable reference signal and a beat frequency signal generated bymixing the output of the gas laser and a multiplied frequency standard,the signal from said phase detector being coupled to control the outputof said current controller to reduce said phase difference, thus tobring about the frequency stabilization of said gas laser.

2. In a gas laser having an output, the frequency of said outputdepending at least to some extent on the amount of current flow throughthe gas discharge of said laser, a currentcontroller for controlling theflow of current through the gas discharge of said laser, a phasedetector for producinga signal which is proportional to the phasedifierence between a stable reference signal and a beat frequency signalgenerated by mixing said laser output with a frequency standard suppliedby a frequency standard generating means, said phase detector beingoperatively connected to control the functioning of said currentcontroller in such a way that the current through said gas discharge iscontrolled to reduce said phase difference, thus to assure a stableoutput frequency of said laser.

3. A phase lock loop utilized in conjunction with a gas laser and afrequency standard, comprising a gas laser having a power supply and adischarge tube in which a gas discharge is produced, and through which acurrent flows, said power supply being capable of supplying variablecurrent through said discharge tube so that the laser output can bevaried in frequency, current controller means for said power supply, forcontrolling the current flow through the laser, phase detector means forgenerating a control signal substantially proportional to the differencein phase of two input signals to said phase detector, with one of theseinput signals being a stable reference, and the other beinga beatfrequency signal out of a mixer means, said mixer means being arrangedto receive the output of said laser and also a signal generated from afrequency standard, and to produce therefrom said beat frequency outputsignal, whose frequency is the difference between the laser signalfrequency and the standard signal frequency, which output signal isdelivered to said phase detector means to generate said control signalwhich is proportional to the phase difference between the phase of saidoutput signal and the phase of the reference signal, said control signalfrom said phase detector means being delivered to said currentcontroller means so as to closely control the current flow through saidgas discharge to reduce said phase difference, thus assuring a frequencystable output.

4. The phase lock loop as defined in claim 3 in which said mixer meansis a point contact multiplier.

5. The phase lock loop as defined in claim 3 in which said frequencystandard is provided by a klystron whose output has been stabilized to afixed reference.

6. The device as defined in claim 5 in which two phase lock loops areutilized for stabilizing said klystron.

l i 1 i i

1. In a gas laser whose output frequency depends on the amount ofcurrent flow through the gas discharge of said laser, a power supply, acurrent controller for controlling the flow of current out of said powersupply and through the gas discharge of said laser, a phase detector forproducing a signal which is proportional to the phase difference betweena stable reference signal and a beat frequency signal generated bymixing the output of the gas laser and a multiplied frequency standard,the signal from said phase detector being coupled to control the outputof said current controller to reduce said phase difference, thus tobring about the frequency stabilization of said gas laser.
 2. In a gaslaser having an output, the frequency of said output depending at leastto some extent on the amount of current flow through the gas dischargeof said laser, a current controller for controlling the flow of currentthrough the gas discharge of said laser, a phase detector for producinga signal which is proportional to the phase difference between a stablereference signal and a beat frequency signal generated by mixing saidlaser output with a frequency standard supplied by a frequency standardgenerating means, said phase detector being operatively connected tocontrol the functioning of said current controller in such a way thatthe current through said gas discharge is controlled to reduce saidphase difference, thus to assure a stable output frequency of saidlaser.
 3. A phase lock loop utilized in conjunction with a gas laser anda frequency standard, comprising a gas laser having a power supply and adischarge tube in which a gas discharge is produced, and through which acurrent flows, said power supply being capable of supplying variablecurrent through said discharge tube so that the laser output can bevaried in frequency, current controller means for said power supply, forcontrolling the current flow through the laser, phase detector means forgenerating a control signal substantially proportional to the differencein phase of two input signals to said phase detector, with one of theseinput signals being a stable reference, and the other being a beatfrequency signal out of a mixer means, said mixer means being arrangedto receive the output of said laser and also a signal generated from afrequency standard, and to produce therefrom said beat frequency outputsignal, whose frequency is the difference between the laser signalfrequency and the standard signal frequency, which output signal isdelivered to said phase detector means to generate said control signalwhich is proportional to the phase difference between the phase of saidoutput signal and the phase of the reference signal, said control signalfrom said phase detector means being delivered to said currentcontroller means so as to closely control the current flow through saidgas discharge to reduce said phase difference, thus assuring a frequencystable output.
 4. The phase lock loop as defined in claim 3 in whichsaid mixer means is a point contact multiplier.
 5. The phase lock loopas defined in claim 3 in which said frequency standard is provided by aklystron whose output has been stabilized to a fixed reference.
 6. Thedevice as defined in claim 5 in which two phase lock loops are utilizedfor stabilizing said klystron.